DESCRIPTION (Adapted from applicant's Description): Many hormones and neurotransmitters exert their effects on cells by raising the intracellular concentration of free Ca2+ ([Ca2+]), thereby inducing the appropriate modulation of a variety of cellular activities including secretion, motility, growth, gene transcription, and differentiation. A key feature of these [Ca2+] signals is an enhanced entry of Ca2+ from the extracellular medium, which plays a pivotal role in shaping the overall Ca2+ signals of cells. The nature and control of this enhanced Ca2+ entry remain controversial, particularly that occurring during the spatially and temporally complex [Ca2+] signals typical of physiological conditions of stimulation. The long-term goal of this research is to understand the molecular basis, mechanism of regulation, and physiological roles of this Ca2+ entry. Although current models for such entry focus on the so-called "capacitative" (or store-operated) mechanism where the emptying of certain intracellular Ca2+ stores is sufficient to activate entry, evidence suggests this may not operate during the complex [Ca2+] signals seen at physiological levels of stimulation. Instead we have recently identified a distinct, noncapacitative Ca2+ entry pathway which appears to be specifically responsible for the entry of Ca2+ during these types of response. This novel pathway is regulated by the receptor-mediated generation of arachidonic acid and involved a Ca2+-selective channel that is entirely distinct from those activated by the capacitative mechanism. We propose to characterize the biophysical nature of the channel involved, the biochemical pathway responsible for its activation, and the role(s) that it plays in shaping and modulating the [Ca2+] signals generated at physiological levels of stimulation. Importantly, the spatial and temporal features of the complex [Ca2+] signals generated under these conditions are known to be critical for the appropriate and targeted activation of specific cellular activities. The key role that this newly identified [Ca2+] entry pathway has in the modulation of these signals indicates that it is likely to be of considerable physiological significance, and an important potential target for future pharmacological manipulation in clinically relevant ways.